The focus of this proposal is to examine the functional and structural organization of a vertebrate spinal network responsible for generating coordinated locomotor activity. To construct a more thorough understanding of network organization it is important to address fundamental issues including the identification of key neuronal components and the characterization of their cellular and synaptic connectivity properties, which help to generate or support the motor output. This study is designed to test specific hypotheses in an attempt to progress our understanding of the organization of the spinal network that produces swimming in larval zebrafish. The specific aims are: 1) To characterize the morphological and neurotransmitter properties of the interneuronal populations marked by Hb9 in the larval zebrafish spinal cord, 2) To test the hypothesis that an identified Hb9 spinal interneuron population or populations participate in driving the locomotor rhythm, and 3) To test the hypothesis that intrinsic membrane properties contribute to the generation of locomotor activity. A combination of neurogenetic, optical imaging and conventional electrophysiological techniques will be used to determine the functional roles specific, identified spinal interneurons play in generating the locomotor drive underlying swimming behavior in zebrafish. Ultimately, the approach outlined in this project will provide a fundamental understanding of the organization of the neural network that generates vertebrate locomotion, which can be used to inform clinical and procedural strategies to treat individuals with spinal cord injuries.